Proximity-induced superconductivity in carbon nanotubes

Single wall carbon nanotubes (SWNT) are model systems for the study of elec tronic transport in one-dimensional conductors. They are expected to exhibi t strong electronic correlations and non-Fermi liquid behavior as suggested by recent experiments. The possibility to induce supercurrents through suc h molecular wires is a challenging question both for experimentalists and t heoreticians. In this paper we show experimental evidence of induced superc onductivity in a SWNT. This proximity effect is observed in a single 1 nm d iameter SWNT, in individual cristalline ropes containing about 100 nanotube s and also on multiwalled tubes. These samples are suspended as strings bet ween two superconducting electrodes (double layer Au-Re, Au-Ta or Sn film) at a distance varying between 100 and 2 000 nm. This allows their structura l study in a transmission electron microscope. When their resistance is low enough, SWNT become superconducting with surprisingly high critical curren ts (in the micro-Ampere range for a single tube of normal state resistance 25 k Omega). This critical current, extensively studied as function of temp erature and magnetic field, exhibits unusual features which are not observe d in conventional Superconducting-Normal-Superconducting junctions and can be related to the strong 1D character of these samples. We also show eviden ce of a huge sensitivity of de transport properties of the tubes to electro magnetic radiation in the radio-frequency range. (C) Academie des sciences/ Elsevier, Paris.